108 research outputs found
Closure statistics in interferometric data
Interferometric visibilities, reflecting the complex correlations between
signals recorded at antennas in an interferometric array, carry information
about the angular structure of a distant source. While unknown antenna gains in
both amplitude and phase can prevent direct interpretation of these
measurements, certain combinations of visibilities called closure phases and
closure amplitudes are independent of antenna gains and provide a convenient
set of robust observables. However, these closure quantities have subtle noise
properties and are generally both linearly and statistically dependent. These
complications have obstructed the proper use of closure quantities in
interferometric analysis, and they have obscured the relationship between
analysis with closure quantities and other analysis techniques such as self
calibration. We review the statistics of closure quantities, noting common
pitfalls that arise when approaching low signal-to-noise due to the nonlinear
propagation of statistical errors. We then develop a strategy for isolating and
fitting to the independent degrees of freedom captured by the closure
quantities through explicit construction of linearly independent sets of
quantities along with their noise covariance in the Gaussian limit, valid for
moderate signal-to-noise, and we demonstrate that model fits have biased
posteriors when this covariance is ignored. Finally, we introduce a unified
procedure for fitting to both closure information and partially calibrated
visibilities, and we demonstrate both analytically and numerically the direct
equivalence of inference based on closure quantities to that based on self
calibration of complex visibilities with unconstrained antenna gains.Comment: 31 pages, 17 figure
Bayesian Accretion Modeling: Axisymmetric Equatorial Emission in the Kerr Spacetime
The Event Horizon Telescope (EHT) has produced images of two supermassive
black holes, Messier~87* (M 87*) and Sagittarius~A* (Sgr A*). The EHT
collaboration used these images to indirectly constrain black hole parameters
by calibrating measurements of the sky-plane emission morphology to images of
general relativistic magnetohydrodynamic (GRMHD) simulations. Here, we develop
a model for directly constraining the black hole mass, spin, and inclination
through signatures of lensing, redshift, and frame dragging, while
simultaneously marginalizing over the unknown accretion and emission
properties. By assuming optically thin, axisymmetric, equatorial emission near
the black hole, our model gains orders of magnitude in speed over similar
approaches that require radiative transfer. Using 2017 EHT M 87* baseline
coverage, we use fits of the model to itself to show that the data are
insufficient to demonstrate existence of the photon ring. We then survey
time-averaged GRMHD simulations fitting EHT-like data, and find that our model
is best-suited to fitting magnetically arrested disks, which are the favored
class of simulations for both M 87* and Sgr A*. For these simulations, the
best-fit model parameters are within of the true mass and within
for inclination. With 2017 EHT coverage and 1\% fractional
uncertainty on amplitudes, spin is unconstrained. Accurate inference of spin
axis position angle depends strongly on spin and electron temperature. Our
results show the promise of directly constraining black hole spacetimes with
interferometric data, but they also show that nearly identical images permit
large differences in black hole properties, highlighting degeneracies between
the plasma properties, spacetime, and most crucially, the unknown emission
geometry when studying lensed accretion flow images at a single frequency.Comment: Accepted to ApJ, 16 pages, 10 figure
The Megamaser Cosmology Project. VII. Investigating disk physics using spectral monitoring observations
We use single-dish radio spectra of known 22 GHz HO megamasers, primarily
gathered from the large dataset observed by the Megamaser Cosmology Project, to
identify Keplerian accretion disks and to investigate several aspects of the
disk physics. We test a mechanism for maser excitation proposed by Maoz & McKee
(1998), whereby population inversion arises in gas behind spiral shocks
traveling through the disk. Though the flux of redshifted features is larger on
average than that of blueshifted features, in support of the model, the
high-velocity features show none of the predicted systematic velocity drifts.
We find rapid intra-day variability in the maser spectrum of ESO 558-G009 that
is likely the result of interstellar scintillation, for which we favor a nearby
( pc) scattering screen. In a search for reverberation in six
well-sampled sources, we find that any radially-propagating signal must be
contributing 10% of the total variability. We also set limits on the
magnetic field strengths in seven sources, using strong flaring events to check
for the presence of Zeeman splitting. These limits are typically 200--300 mG
(), but our most stringent limits reach down to 73 mG for the galaxy
NGC 1194.Comment: Accepted for publication in Ap
Multi-frequency Black Hole Imaging for the Next-Generation Event Horizon Telescope
The Event Horizon Telescope (EHT) has produced images of the plasma flow
around the supermassive black holes in Sgr A* and M87* with a resolution
comparable to the projected size of their event horizons. Observations with the
next-generation Event Horizon Telescope (ngEHT) will have significantly
improved Fourier plane coverage and will be conducted at multiple frequency
bands (86, 230, and 345 GHz), each with a wide bandwidth. At these frequencies,
both Sgr A* and M87* transition from optically thin to optically thick.
Resolved spectral index maps in the near-horizon and jet-launching regions of
these supermassive black hole sources can constrain properties of the emitting
plasma that are degenerate in single-frequency images. In addition, combining
information from data obtained at multiple frequencies is a powerful tool for
interferometric image reconstruction, since gaps in spatial scales in
single-frequency observations can be filled in with information from other
frequencies. Here we present a new method of simultaneously reconstructing
interferometric images at multiple frequencies along with their spectral index
maps. The method is based on existing Regularized Maximum Likelihood (RML)
methods commonly used for EHT imaging and is implemented in the eht-imaging
Python software library. We show results of this method on simulated ngEHT data
sets as well as on real data from the VLBA and ALMA. These examples demonstrate
that simultaneous RML multi-frequency image reconstruction produces
higher-quality and more scientifically useful results than is possible from
combining independent image reconstructions at each frequency.Comment: 25 pages, 15 figures. Accepted to Ap
- …